The present invention relates to voltage comparators and, more particularly, to voltage comparator circuits having hysteresis for producing output transitions when an applied voltage exceeds a first voltage threshold and then falls below a lower second voltage threshold.
The prior art is replete with voltage comparators which produce output transitions in response to an unknown voltage input signal exceeding a predetermined voltage level and then falling below this level. Many such comparators include circuitry for producing hysteresis. In these latter circuits, the comparator generally comprises a differential amplifier having a pair of inputs and an output at which the output transitions occur. The input signal is applied to a first one of the inputs of the differential amplifier while a first threshold voltage is applied to the other input. A hysteresis producing circuit is responsive to the input signal exceeding the first threshold voltage for establishing a second threshold voltage at the other input the magnitude of which is less than the first threshold voltage. In this manner, once the output of the comparator is tripped the input signal must decrease below the value of the second threshold voltage before the output of the comparator is once again tripped which places it in its original state. Hysteresis is produced due to the two different voltage levels required to trip the comparator between the two operating states. Hysteresis is required for voltage comparators used in a high noise environment where large voltage transient signals are not uncommon. When the input signal is in the vicinity of the first threshold voltage noise can cause the input to vary above and below the threshold voltage which would otherwise cause the comparator to oscillate rapidly between its two operating output states absence hysteresis. However, with hysteresis, if the first threshold voltage is well defined, as soon as the input signal exceeds the threshold voltage the comparator will change states causing the first voltage level to decrease to the second voltage level. This inhibits noise transients from producing the oscillations described above as is understood.
In one prior art voltage comparator a multicollector PNP transistor is utilized as a source of currents to the differential amplifier and the hysteresis producing circuit. In operation, as the input signal exceeds the first threshold voltage, the differential amplifier, which comprises first and second transistors, switches states such that current is sourced from one of the collectors of the multicollector transistor to supply current drive to a additional transistor included in the hysteresis producing circuit. The additional transistor is turned on causing the first threshold voltage level to be reduced by decreasing the voltage supplied at the base of the second transistor of the differential amplifier. Hence, the input signal must fall below this second threshold voltage level to cause the output of the voltage comparator to switch back to its original state.
A problem with the above described prior art voltage comparator is that the first threshold voltage is not well defined and does not remain fixed due to the mentioned second transistor being driven into saturation whenever the magnitude of the input signal is less than the first threshold voltage. Because the second transistor is in a saturated condition saturation current is emitted from its base, as is well understood, which is injected into the substrate of the integrated voltage comparator circuit due to the inherent parasitic PNP transistor formed at the collector of the second transistor. This lowers the first threshold voltage level. As the input signal increases in value, the second transistor tends to become less saturated. This allows the first threshold voltage level to increase as less saturation current is injected into the substrate. Hence, the value of the first threshold voltage moves up with the increasing input signal until such time that the second transistor is turned completely off by the input signal value exceeding the maximum permissible value of the first threshold voltage.
Thus, the prior art suffers in that the threshold voltage level at which the voltage comparator trips states is ill defined. This can permit undesirable oscillations to occur as the output is tripped between operating states if noise transients should occur at the input of the voltage comparator.
Hence, a need exists for an improved integrated voltage comparator circuit having hysteresis and a well defined trip point.